Colby Instruments, Inc. provides characteristic--impedance-correcting standard or reference lines capable of realizing adjustable delays (group time delays or delay times) as delay lines PDL 10A, PDL 20A. Within a fairly narrow range of values, the delay times can be selected arbitrarily.
These known delay lines work exclusively with coaxial lines which can be extended or contracted and of the so-called variable length or telescoping line ("trombone") type or with coaxial relays. The fabrication and adjustment of such systems is extraordinarily expensive and the damping is frequency-dependent.
For diversity systems in radio relay or microwave technology, delay time differences have had to be tolerable (see IFC Publication 835, Part 2, Section 7, FIG. 2) and delay was provided by manually adjustable coaxial cable sections which were introduced in the measuring system at lengths corresponding to the desired group time delay. Here, however, as in the previously described system, apart from the desired group time delay, there was an accompanying undesired frequency dependence of the damping. Such systems are also costly and the use of the system is time-consuming.